A preliminary investigation has found that friendly fire was likely to blame in a shooting that killed one federal agent and wounded another on the Arizona-Mexico border, the FBI said Friday. The incident has re-ignited political debate over security on the border.

"There are strong preliminary indications that the death of United States Border Patrol Agent Nicholas J Ivie and the injury to a second agent was the result of an accidental shooting incident involving only the agents," FBI Special Agent in Charge James L Turgal Jr said in a statement.

Turgal did not elaborate on the agency's conclusions but said the FBI was using "all necessary investigative, forensic and analytical resources in the course of this investigation" into the incident, which occurred on Tuesday, about five miles north of the border near Bisbee.

Ivie was shot and killed after he and two other agents responded to an alarm triggered by a sensor aimed at detecting smugglers and others entering the US illegally. One of the other agents was shot in the ankle and buttocks and was released from the hospital after surgery. The third agent was uninjured.

The Cochise County Sheriff's Office, which is assisting the FBI, said federal investigators had used ballistic testing to determine the shootings were likely the result of so-called friendly fire among the agents.

Jeffrey D Self, commander of Customs and Border Protection's Joint Field Command-Arizona, said investigators were making progress but noted that despite initial findings that the shootings appeared to have been accidental, it did not diminish the fact that Ivie "gave the ultimate sacrifice and died serving his country".

"The fact is the work of the Border Patrol is dangerous," Self said, during a news conference in Tucson.

While federal authorities declined to offer details of the shooting, George McCubbin, president of the National Border Patrol Council, said the three agents split up as they investigated the sensor alarm, noting that all three fired their weapons.

"Coming in from different angles, that is more than likely how it ended up happening," McCubbin told The Arizona Republic.

A Mexican law enforcement official said on Thursday that federal police had arrested two men who may have been connected to the shootings. The official, who spoke on condition of anonymity because he was not authorized to release the information, said it was unclear if there was strong evidence linking the men to the case.

It was unclear on Friday whether the two men remained in custody or were still being considered part of the investigation.

After a meeting of border governors Friday in Albuquerque, New Mexico, Arizona governor Jan Brewer stood by the criticism she leveled earlier this week in response to the shootings in which she said a political stalemate and the federal government's failures have left the border unsecured and Border Patrol agents in harm's way.

"It's the federal government's responsibility to secure our border, and they need to do that, and then we can deal with all the other issues that have come about because our border hasn't been secured," said Brewer, who plans to attend Ivie's funeral on Monday in Sierra Vista.

The Border Patrol could not immediately comment on the frequency of friendly fire shootings at the agency, but such incidents appeared to be extremely rare, if they've ever occurred at all.

"I know of absolutely none in the past, and my past goes back to 1968," Kent Lundgren, chairman of the National Association of Former Border Patrol Officers said, citing the year he joined the agency. "I'm not saying it never happened. I'm just saying I've never heard of it."

Also on Friday, Homeland Security Secretary Janet Napolitano traveled to Arizona to express her condolences to Ivie's family and meet with authorities.

Ivie's death was the first fatal shooting of an agent since a firefight with Mexican bandits that killed the US Border Patrol Agent Brian Terry in December 2010 and spawned congressional probes of a botched government gun-smuggling investigation.

Terry's shooting was later linked to that "Fast and Furious" operation, which allowed people suspected of illegally buying guns for others to walk away from gun shops with weapons, rather than be arrested. Authorities intended to track the guns into Mexico. Two rifles found at the scene of Terry's shooting were bought by a member of the gun-smuggling ring being investigated.

Critics of the operation say any shooting along the border will raise the specter that those illegal weapons are still being used.

Twenty-six Border Patrol agents have died in the line of duty since 2002.